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Flashcards · 9700 · A Level Topic 16
Inheritance (A Level) flashcards
19 cards on A Level Topic 16, subtopics 16.1 to 16.3. Pick a subtopic or study them all.
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16.1 Passage of information from parents to offspring
- Haploid, diploid and homologous pair
- Haploid (n): one set of chromosomes. Diploid (2n): two sets, one from each parent. Homologous pair: one maternal and one paternal chromosome with the same genes at the same loci; alleles may differ.
- Why must gametes be haploid?
- So that fertilisation restores the diploid number instead of doubling the chromosome number each generation.
- Anaphase I vs anaphase II
- Anaphase I: homologous chromosomes move to opposite poles; centromeres stay intact. Anaphase II: centromeres divide and sister chromatids move to opposite poles.
- Three sources of genetic variation in sexual reproduction
- Crossing over between non-sister chromatids in prophase I; random orientation (independent assortment) of homologous pairs in metaphase I and of chromatids in metaphase II; random fertilisation.
- How many chromosome combinations can independent assortment give?
- 2ⁿ, where n is the number of homologous pairs. For humans (n = 23), 2²³ ≈ 8.4 million.
16.2 The roles of genes in determining the phenotype
- Gene vs allele
- Gene: a length of DNA at a locus coding for a polypeptide. Allele: one version of that gene.
- Dominant vs codominant alleles
- A dominant allele masks the other allele in a heterozygote. Codominant alleles are both expressed in the heterozygote (for example HbA and HbS).
- Expected phenotype ratios: monohybrid and dihybrid crosses
- Aa × Aa: 3 : 1 (1 : 2 : 1 with codominance). Aa × aa: 1 : 1. AaBb × AaBb (unlinked): 9 : 3 : 3 : 1. AaBb × aabb (unlinked): 1 : 1 : 1 : 1.
- Autosomal linkage vs epistasis
- Linkage: two genes on the same autosome are inherited together, so parental classes are in excess and recombinants few. Epistasis: one gene affects the expression of another, modifying the ratio.
- Chi-squared test: steps
- Null hypothesis: no significant difference between observed and expected. χ² = Σ (O − E)² ÷ E. Degrees of freedom = classes − 1. Compare with the critical value at p = 0.05 (1 df 3.84; 2 df 5.99; 3 df 7.82). Below it: not significant.
- How does the TYR gene affect phenotype?
- TYR codes for tyrosinase, which converts tyrosine to melanin. A faulty allele gives albinism (little or no melanin); autosomal recessive.
- How does the Le/le gene affect stem height in peas?
- Le codes for a functional enzyme that makes active gibberellin (GA₁ from GA₂₀). le codes for a non-functional enzyme, so lele plants have little active gibberellin, little stem elongation and are dwarf.
- Inheritance of haemophilia and Huntington's disease
- Haemophilia (F8, factor VIII): X-linked recessive. Huntington's disease (HTT, huntingtin): autosomal dominant, caused by an expanded CAG repeat.
16.3 Gene control
- Structural gene vs regulatory gene
- Structural gene: codes for a protein used in the cell, such as an enzyme. Regulatory gene: codes for a protein that controls other genes, such as the lac repressor.
- Inducible vs repressible enzyme
- Inducible: made only when its substrate is present (β-galactosidase). Repressible: production stops when its product builds up (tryptophan synthesis enzymes).
- The lac operon when lactose is absent
- The repressor protein (from lacI) binds to the operator, blocking RNA polymerase, so lacZ, lacY and lacA are not transcribed.
- The lac operon when lactose is present
- Lactose (as allolactose) binds to the repressor, which changes shape and leaves the operator. RNA polymerase transcribes lacZ, lacY and lacA: β-galactosidase, lactose permease and transacetylase are made.
- Transcription factor
- A protein that binds to DNA and increases or decreases the rate of transcription of a gene (in eukaryotes).
- How does gibberellin switch on growth genes?
- DELLA proteins bind transcription factors and stop them promoting transcription. Gibberellin binds its receptor, causing DELLA proteins to be broken down; the transcription factors are released and transcription of growth genes increases.